Gas turbine engines include one or more compressors for pressurizing a working medium fluid, typically ambient air, which flows through a longitudinally extending compressor flow path. Under some operating conditions, it is desirable to temporarily moderate the pressure at the discharge end of the compressor to prevent or recover from compressor stall or other aerodynamic instabilities. Pressure moderation is usually effected by opening a compressor bleed valve that diverts a portion of the pressurized fluid from the discharge end of the compressor flow path into a lower pressure region. Such compressor bleed air may also be used to drive aircraft accessories and provide cooling within the aircraft.
An exemplary compressor bleed system includes a stationary bleed duct case with sealing faces and a moveable valve ring containing a pair of resilient seal members adhesively bonded or clamped into respective channels on longitudinally facing surfaces of the ring. A series of circumferentially distributed passages extend through the bleed duct case to join the compressor flow path to a surrounding annular chamber. A set of pins extends radially from the valve ring, and each pin includes a roller (a cam roller and a damper roller) that engages carved slots on a bracket attached to the bleed duct case wall. A single bell crank for operating the valve ring is mounted on a bell crank support bracket by a bell crank pivot. Input and output arms of the bell crank are connected respectively to an actuator and to the valve ring.
In operation, the actuator rotates the bell crank about the bell crank pivot so that the bell crank, in turn, drives the valve ring in a spiral motion (translating along and rotating about the engine core axis), positioning the valve ring to cover or uncover the passages. When the bell crank is rotated to force the valve ring seals against the duct sealing faces, air is sealed into the low pressure compressor flowpath. The bell crank connects to the valve with a first arm and connects to a connecting link with a second arm. The bell crank is mounted to a stiff case structure, such as the intermediate compressor case, with a bracket.
For most applications there is sufficient height and axial space to package these components. However, certain engine configurations have a limited amount of space to fit a bleed valve actuator over the intermediate compressor case, and there is not always a sufficient amount of space to couple a traditional bell crank to actuate the valve ring.